There have been significant developments in RF excited, diffusion cooled, sealed-off slab lasers. These lasers typically include a housing containing a laser gas. A pair of elongated, planar electrodes are disposed with the housing. The electrodes are spaced mounted in face to face relationship and spaced apart to define a narrow gap corresponding to the discharge region. Mirrors are positioned at the ends of the electrodes to define the resonator. A basic slab laser design is disclosed in commonly owned U.S. Pat. No. 5,140,606 incorporated herein by reference.
In some of the slab laser designs, it has been found desirable to include dielectric members along the edges of the electrodes. One basic design can be found in an article by Xin, et al., “RF Discharge Excited Diffusively Cooled KW CO Slab Waveguide Laser with a Three Mirror Resonator” (Appl. Phys. Letters, p 1369, 6 Sep. 1999). FIG. 1 herein corresponds in some respects to the electrode structure illustrated in FIG. 2 of the Xin paper. The electrode structure includes a pair of rectangular planar electrodes 1 and 2. Electrode 1 is the hot electrode and electrode 2 is the ground electrode. The electrodes are typically made from aluminum. An opposed pair of dielectric plates 3 and 4 are positioned in the gap between the electrodes. The outer edges of the plates extend beyond the edges of the electrodes. The inner edges of the plates extend part of the way into the discharge gap. The region between the electrodes and the inner edges of the plates defines a slab shaped discharge region 5. The plates and the electrodes function to enclose the discharge except at the two ends thereof. The plates are typically made from an alumina ceramic material.
In a preferred embodiment found in some prior art designs, the ends of the electrodes can be provided with ceramic blocks 6 and 7 that extend the optical waveguide between the shorter hot electrode and the longer ground electrode (see, commonly owned U.S. Pat. No. 5,216,689 incorporated herein by reference). The ceramic blocks act as the dielectric laser waveguide extensions to increase the efficiency of the laser by reducing optical losses caused by the laser mode leaving the waveguide aperture, propagating a distance in free space, and then reflected back into the waveguide aperture. The longer the distance the laser mode propagates in free space, the higher the mode matching losses become. Extending the waveguide as shown in FIG. 1 reduces such losses.
More importantly is the fact that extending the waveguide with a dielectric also increases the laser lifetime by keeping the discharge away from the optical resonators mirrors, thereby preventing damage to the mirrors from the UV radiation generated by the discharge. With the ceramic extended waveguide, the discharge essentially stops at the end of the hot electrodes, which is shorter than the ground electrode.
The assembly is excited from an RF power source 8. The power source excites the discharge creating laser light. Details concerning the preferred method of mounting the electrodes, the cooling of the electrodes and their placement within a hermetically sealed laser tube housing are described in U.S. application Ser. No. 12/079,296, filed Mar. 26, 2008, incorporated herein by reference.
Enclosing the discharge with dielectric strips helps eliminates acoustic coupling between the discharge region and the structures outside the discharge regions. We have observed that as the laser discharge pulse repetition frequency (PRF) passed thorough an acoustic resonance region of the laser tube housing structure, regions within the discharge began to vary in intensity and these regions began to move around. Such discharge non-uniformity affected both the output beam pointing and amplitude stability. We believe this beam movement is caused by the alternating pressure gradients associated with the acoustic resonances. We observed that enclosing the discharge between the two dielectric strips prevented acoustic coupling of the pulsed discharge with the structures outside the discharge regions thereby minimizing or preventing the development of some of the acoustic resonances affects. This approach improved pointing stability as a function of varying RF pulsed repetition frequency and pulsed duty cycle. Reducing the acoustic resonances within the discharge region also improved the discharge stability and therefore reduced the variation in the power output of the laser.
Although the structure shown in FIG. 1 worked, it was found that having the discharge interact with the inner edges of the plates can cause problems with lifetime and laser stability. The subject invention is intended to address those concerns.
Further information about slab lasers and dielectric spacers can be found in commonly owned U.S. Pat. Nos. 7,260,134 and 7,263,116, both of which are incorporated herein by reference.
Further advantages of the subject invention will be discussed below in conjunction with the drawings in which: